1. Field of the Invention
This invention relates to adhesive compositions. This invention also relates to articles comprising adhesive compositions.
2. Description of the Related Art
Modified polymers that have been functionalized (typically by maleic anhydride grafting) with reactive groups are widely used as extrusion and lamination adhesives, particularly as adhesive layers in multilayer composite structures. Maleic anhydride-grafted polymers (often referred to as “maleated” polymers) are also commonly used in pipe coatings and building panels.
Maleic anhydride-grafted polymers include maleated polyethylene, maleated polypropylene, maleated styrene-ethylene-butene-styrene triblock copolymer, and maleated polybutadiene. Ethylene/alky acrylate/carbon monoxide terpolymers, ethylene/vinyl acetate copolymers ethylene propylene rubbers can also be modified by maleic anhydride grafting. An example of a maleic anhydride modified linear high density polyethylene is a product sold under the trademark Polybond® 3009 available from Crompton Corporation. Similar maleated polyolefins are sold under the trademark Fusabond® available from DuPont.
U.S. Pat. Nos. 5,516,583; 4,861,676; 4,861,677; 4,552,819 and 5,965,255 disclose adhesive compositions. U.S. Pat. No. 5,225,482, discloses an adhesive composition comprising an ethylene-vinyl acetate copolymer, a styrene polymer resin, a graft-modified polyethylene, a polystyrene elastomer and an ethylene-olefin copolymer. PCT Patent Publication WO01/18141 discloses an adhesive composition comprising polyolefin, a functional polyolefin, a polystyrenic composition, and an elastomer.
Conventional modified polyolefins sacrifice either bonding strength to the metal or to the polyolefin, or do not activate at low temperatures when bonding to metals or polyolefins. Furthermore, adhesive compositions that activate at low temperatures can be very soft and tacky materials such that the handling of these compositions as free films in thermal laminating processes can be problematic and can require very costly release films to prevent blocking. Also, adhesives can significantly lose their bond strength over time after they are put into service. This phenomenon is referred to as “age-down” in the industry.
The production and use of maleated polyolefins and other maleic anhydride-grafted copolymers is not trouble-free. For example, it can be difficult to reach a high level of functionalization in a maleated polyolefin by grafting because the required reaction conditions can cause undesirable side reactions, such as crosslinking and chain scission. For this reason, grafting of maleic anhydride at a concentration of greater than about two weight % can be difficult and troublesome.
It can be desirable to use adhesive compositions that do not have the problems associated with high levels of grafting in maleated polyolefins yet provide superior adhesive performance.
Ethylene copolymers that include reactive functional groups such as maleic anhydride can be readily obtained by direct copolymerization using a high-pressure free radical process. Such a high-pressure is described, for example, in U.S. Pat. No. 4,351,931.
PCT patent Application publication WO2003/099930 A1 describes polyamides toughened by an impact modifier composition comprising a copolymer prepared from ethylene and a functional comonomer, wherein the functional comonomer is included in the range of from about 3 to about 15 weight % in the copolymer.